1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, more particularly to an electrophotographic photosensitive member of high durability excellent in humidity resistance and mechanical strength.
2. Related Background Art
An electrophotographic photosensitive member is required to have prescribed sensitivity, electrical characterictics and optical characteristics corresponding to the electrophotographic process to be applied. Further, in a photosensitive member which is used repeatedly, since electrical and mechanical external force such as corona charging, toner development, transfer onto paper, cleaning treatment, etc., is directly applied onto the surface layer of the photosensitive member, namely the layer which is the remotest from the substrate, durability to those forces is required.
More specifically, durability to generation of abrasion or damage by the friction of the surface or to deterioration of the surface by ozone generated during corona charging under humid conditions is required.
On the other hand, there is also the problem of toner attachment onto the surface layer by repeated development of toner and cleaning, and to cope with this problem, improvement of the cleaning characteristic of the surface layer has been demanded.
In order to satisfy the characteristics required for the surface layer as mentioned above, various methods have been investigated. Among them, the means of dispersing fluorine type resin powder into the surface layer is effective. By dispersion of fluorine type resin powder, the frictional coefficient of the surface layer is lowered to act on improvement of the cleaning characteristic as well as improvement of durability to abrasion damage.
Also, since water-repellent property and mold-release property of the suface layer can be improved, it is also effective against prevention of the surface deterioration and highly humidity conditions.
However, in fluorine resin powder dispersion, problems are involved in its dispersibility and agglomerating tendency, and since it is difficult to form a uniform and smooth film, the surface layer obtained could not avoid having image defects such as image irregularities or pinholes.
Also, although some binder resins or dispersing aids can disperse uniformly fluorine type resin powder to form a smooth film, in most cases, due to having hydroxyl groups, carboxyl groups, ether bonds, etc., carrier traps are formed particularly under high temperature and highly humid conditions to cause deterioration in electrophotographic characteristics. Thus, under the present situation, no practically available binder resin or dispersing aid can be found.